This invention relates to a method for controlling an injection molding machine, and more particularly to a method for maintaining the spacing between parting planes of metal mold halves of an injection molding machine at a predetermined value.
In this type of the injection molding machine although the metal mold halves are held closed during molding by a clamping force they tend to separate when the injection pressure exceeds the clamping force, and when the mold is deformed due to nonuniform injection of the resin into the mold. Separation of the mold halves results in the variation of the quantity of the resin injected into the mold so that it is desirable to limit the spacing between the mold halves within a permissible range.
The magnitude of the spacing between parting planes of metal mold halves (for brevity, hereinafter merely called "the spacing between mold halves") of an injection molding machine during its injection stroke varies dependent upon the quantity of the resin to be injected into the metal mold, the injection speed and the injection pressure.
Referring now to FIG. 1, it is now assumed that an injection stroke S is divided into four sections A, B, C and D and that the movement of an injection plunger including a screw 8 is controlled such that it is moved at different speeds v.sub.1, v.sub.2, v.sub.3 and v.sub.4 in sections A, B, C and D, respectively, while the plunger is moved over the injection stroke S. The spacing .DELTA.l between the mold halves during this operation varies dependent upon the quantity of the resin to be injected, that is the length of the injection stroke S determined by an injection stroke position detector to be described later, the lengths of respective sections A, B, C and D, or the injection speeds v.sub.1, v.sub.2, v.sub.3 and v.sub.4 in respective sections A, B, C and D. As the magnitudes of injection speeds v.sub.1 -v.sub.4 cause variations in the cooling and solidifying conditions of the resin filled in the metal mold, the apparent viscosity of the resin in the metal mold is varied thereby varying the dynamic pressure applied to the inner surface of the metal mold by the resin. This also causes variation in the spacing between the mold halves. Let us denote by K.sub.1 the upper limit of a permissible range of the spacing between mold halves which is necessary to produce satisfactory products and by K.sub.2 the lower limit of the range. Satisfactory products can be obtained where a condition K.sub.1 .gtoreq. .DELTA.l .gtoreq. K.sub.2 is satisfied. Where K.sub.1 &lt; .DELTA.l, the molded product will be overpacked whereby flashings are formed on the molded product whereas when K.sub.2 &gt; .DELTA.l .gtoreq. 0 the products will have sink marks, or improper dimensions. Even when such sink marks or improper dimensions do not result, the physical properties of the products will be inferior to those of the satisfactory products molded under normal conditions. For example, the weight of the products increases as the spacing increases.
In order to prevent the formation of such rejects, according to an established practice, skilled operators set the operating condition in accordance with the type of the resin, and the configuration of the molded product by relying upon their experience or feeling, and after a number of trial operations they commence regular or normal operations. Further, during the normal operation, should rejects be formed, the operator sets a new operating condition again by relying upon his experience and feeling and repeats a number of trial operations before he can find out the best operating condition. Such method of operation requires much time and labor and waste of raw material during trial operations, and hence is inefficient.